The meaning of "Piss" in various phrases and sentences

This is a vulgar variation of "to bring a knife to a gunfight", which means "to start a fight without being properly prepared". This sounds really intense and it basically means "You're trying to start a fight with me, but there's absolutely no way that this is going to go down well for you because you're extremely weak and I could kick your ass into oblivion without exerting even the most minimal amount of effort. Bitch."

This is a rather rude expression nto to be used formally and best used only with friends or people you know well. It means something is easy. "I repaired our kitchen tap last night. As I had the right spanner and washer, it turned out to be a piece of piss to sort it".

Yes, it is just a racial slur, like your dictionary says, but in this political climate racial slurs are a very serious issue. Offensive words lead to irrational hatred of people of a certain race, which leads to violence.

'Wish' means that you have already met her. You said that you have never met her and hope that you never will. That is implying a future action. If you wanted to say wish, then you should have said, "I have met my step-mother, and I wish that I had never met her." In this instance, 'wish' is speaking of the past, 'hope' is speaking of the future.

not bad at all! but I would just add some adjustments here and there to make it sound more natural. I'll add my edit here:
In my opinion, people fall into two categories: those who hate spoilers and those who don't care. Whichever group you are belong to depends on what kind of movies you like.
The first group watches movies based on plot twists and everything revolves around that. Apparently, if you tell them what'll happen, it'll piss them off. What's the point of watching the movie if you know that the butler is a murderer?
The second group prefers movies with interesting dialogues, good acting, and good cinematography. Basically, even if they know what happens next they still can enjoy the movie. I'm in the second group, I have a lot of favorite movies I've seen many times. I know each world that the characters say and each action they do but it makes me even more excited and reinforces the impression of some events in the movies.
Enjoy good movies and don't be afraid of spoilers!

In my opinion, people fall into one of two categories; those who hate spoilers and those who don't care, and which group you belong to depends on what kind of movies you like.
The first group watches movies that revolve around plot twists. Apparently, if you tell them what'll happen, it'll piss them off. What's the point of watching the movie if you already know that the butler is a murderer?
The second group prefers movies with interesting dialogues, good acting and skilled camera work. Basically, even if they know what happens next, they can still enjoy the movie.
I'm in the second group. I have a lot of favorite movies I've seen many times. I know every character's lines and actions, but this only gets me even more excited and reinforces my impression of certain events in the movies.
Love good movies and don't be afraid of spoilers!